Avoid SpotBugs warning

 SpotBugs is concerned that getCanonicalHostName() may return null, although the JavaDocs indicate that it will return the IP address as a String if unable to determine the FQDN.  The only circumstance I can imagine where it would return null is if the IP address is null, which would mean a reverse lookup wouldn't work either.

Author: Steve Vaughan Jr

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/net/DNSDomainNameResolver.java

@@ -50,7 +50,7 @@ public String getHostnameByIP(InetAddress address) {
     }
     // Protect against the Java behaviour of returning the IP address as a string from a cache
     // instead of performing a reverse lookup.
-    if (host.equals(address.getHostAddress())) {
+    if (host != null && host.equals(address.getHostAddress())) {
       LOG.debug("IP address returned for FQDN detected: {}", address.getHostAddress());
       try {
         return DNS.reverseDns(address, null);